1. Field of the Invention
The present invention relates to a semiconductor switching device such as a thyristor, transistor, diode and complex bipolar devices, and a method of controlling a carrier lifetime in a semiconductor switching device.
2. Description of the Background Art
It is known in the art to control a carrier lifetime in a semiconductor switching device by irradiating the semiconductor switching device with a radiation, such as an electron beam, a proton beam, neutron beam, alpha rays, charged particle beam, and gamma rays, to cause radiation defects, or lattice defects in a wide sense. An example, in which a buried gate type static induction thyristor (hereinafter referred to as an SI thyristor) is irradiated with a radiation, will be described.
FIG. 1 is a cross-sectional view schematically showing the main portion of a structure of a conventional SI thyristor which has radiation defects caused by irradiation with a radiation. FIG. 2 is a view showing a process of irradiating an SI thyristor with a radiation.
Referring to FIG. 1, the SI thyristor comprises an N type semiconductor substrate 1, an anode region 2 which is formed on the back surface of the semiconductor substrate 1 by diffusing P type impurities, a cathode region 3 which is formed on the top surface of a jut of the semiconductor substrate 1 by diffusing N type impurities, and gate regions 4 which are, formed on step portions of the semiconductor substrate 1 by diffusing P type impurities. A main current 9 flows through a channel region 5, which is surrounded by the gate regions 4.
This SI thyristor is irradiated with a radiation 10 such as an electron beam to cause radiation defects 7 in the semiconductor substrate 1, as shown in FIG. 2. The radiation defects 7 are uniformly distributed all over the irradiated portion of the semiconductor substrate 1, since the radiation defects 7 are caused by the interaction of the radiation 10 and atoms and/or vacancy forming the semiconductor substrate 1. In FIG. 1 and FIG. 2, however, the radiation defects 7 are shown by only some cross marks for the convenience of illustration.
The operation of the SI thyristor, in the condition that a carrier lifetime is uniformly decreased all over the semiconductor substrate 1 due to the radiation defects 7, will now be roughly described. In an ON state, the main current 9 flows from the anode region 2 to the cathode region 3 through the channel region 5 against the interruption of the radiation defects 7, while many carriers 8 are trapped by the radiation defects 7. At the moment of a turn OFF of the SI thyristor, a number of carriers remain in the semiconductor substrate 1. If the lifetime of the carriers is long, the substantial turn OFF time of the SI thyristor becomes long. However, the SI thyristor shown in FIG. 1 has a number of radiation defects 7 which serve as a lifetime killer to the carriers, and hence the turn OFF time of the SI thyristor is decreased because a tail current and, therefore, a tail time is decreased. Thus, the switching loss of the SI thyristor in a turn OFF transition is greatly decreased by irradiating the SI thyristor with the radiation 10 to control a carrier lifetime.
However, such uniform irradiation of the semiconductor substrate 1 with the radiation 10 causes a number of radiation defects 7 in the channel region 5 through which the main current 9 flows, so that the forward voltage of the SI thyristor in an ON state is disadvantageously increased, mainly due to the decrease of a carrier lifetime in the channel region 5. The increase of the forward voltage results in the increase of a power loss in an ON state. Especially in a high breakdown voltage device such as a device having a higher breakdown voltage than 600 V, such increase of the forward voltage causes unacceptable ON state power loss and exothermic device destruction, since such a device generally has a substrate having a high specific resistance.
Further, at the initial stage of a turn ON transition, carriers must progress against the radiation defects 7, in other words, the main current 9 must flow in the substrate having higher specific resistance but shorter minority carrier lifetime caused by the radiation defects 7, and hence the turn ON time of the SI thyristor is increased. This results not only in increase of a power loss but also in decrease of an operation frequency. Although, in a bipolar type device, a substrate of higher specific resistance results in lower ON voltage because a conductivity modulation can be easily caused due to low impurity concentration and long carrier lifetime, ON voltage is increased if the specific resistance of a substrate is increased as the result of decrease of a carrier lifetime due to irradiation.
If the SI thyristor is uniformly irradiated with a lot of radiation 10 to perform a strong lifetime control, a leak current in an OFF state is increased due to the occurrence of defects in a passivation film (not shown) and/or the deterioration of a surface condition of the substrate 1, and a main breakdown voltage is decreased due to the increase of a leak current at the exposed edges of P-N junctions defined by the substrate 1 and the gate regions 4.
In an MOS type transistor and thyristor, such as MOSFET and an Insulated Gate Bipolar Transistor (hereinafter referred to as an IGBT), MOS Gate SI Thyristor (hereinafter referred to as an MOS-SITh), MOS Gate GTO Thyristor (hereinafter referred to as an MOS-GTO), MOS Controlled Thyristor (hereinafter referred to as an MCT), Emitter Switched Thyristor (hereinafter referred to as an EST), MOS Assisted Gate Thyristor (hereinafter referred to as an MAGT), and other kind of MOS Gate devices, a gate portion includes an insulator such as a silicon oxide film. Therefore, if the MOS type device is irradiated with a radiation strongly to control a carrier lifetime, various surface states, which are induced in the interface of the gate portion and the substrate, make a gate controlling main current flow through beneath a gate difficult, in addition to the problems as hereinbefore described.
Thus, the lifetime control of a semiconductor switching device by the irradiation of a radiation such as an electron beam has the advantage of decrease of a turn OFF time and the disadvantages of increase of a turn ON time as well as a forward voltage, and in some cases decrease of a breakdown voltage and especially in MOS devices to lose a controllability of MOS Gate. The advantage and disadvantages are in a trade-off relationship, so it is difficult to appropriately accommodate the trade-off relationship for particular use of the semiconductor switching device.
A method of partly recovering deteriorated characteristics of semiconductor switching device caused by irradiation is annealing process, which is conventionally and widely used to recover some kinds of crystal defects. Through the annealing process, the deteriorated characteristics are uniformly recovered, since a whole semiconductor switching device is only put into a uniformly high temperature condition during the anneal processing. Therefore, point of view to improve the trade-off relationship between a turn-off time and on-state voltage, this annealing process is equivalent to choose optimum quantity of irradiation of a radiation to implement a device having desired characteristics, and hence the problems as hereinbefore described cannot be solved in essence through the annealing process.
According to the first aspect of the present invention, a semiconductor switching device has a semiconductor layer through which a main current flows. This semiconductor layer is so structured that a carrier lifetime in the semiconductor layer is nonuniform in accordance with a predetermined distribution of the carrier lifetime.
According to the second aspect of the present invention, a method of controlling a carrier lifetime in a semiconductor switching device, having a semiconductor layer through which a main current flows, comprises the steps of preparing a semiconductor switching device and a radiation source for emitting a radiation, and irradiating the semiconductor switching device with the radiation emitted from the radiation source to cause radiation defects distributed in the semiconductor layer, in predetermined space and density distributions of the radiation defects.
Accordingly, it is an object of the present invention to provide a semiconductor switching device and a method of controlling a carrier lifetime in a semiconductor switching device, in which turn OFF characteristics of a semiconductor switching device can be improved without causing any unacceptable disadvantages for other characteristics.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.